Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)5 in solution

Abstract

frontier-orbital interactions with atom specificity. We anticipate that the method will be broadly applicable in the chemical sciences, and complement approaches that probe structural dynamics in ultrafast processes.In our experimental set-up (Figure 1a), the valence electronic structure of Fe(CO) 5 is probed with femtosecond-resolution resonant inelastic x-ray scattering (RIXS) at the Fe L 3 -edge (Fe experiments. This triplet arises from a singlet state with a time constant of 300 fs, consolidating the notion 6 that sub-ps intersystem crossing appears to be common in the excited-state dynamics of transition-metal complexes 7,[22][23][24] . The persistence of the triplet Fe(CO) 4 ( 3 B 2 ) up to our maximum time delay of 3 ps is consistent with it undergoing a slow, spin-forbidden reaction with intersystem crossing to a solvent-complexed singlet state on the 50-100 ps time scale 4,5, 25 . However, the observed branching on a sub-ps time scale into the competing and simultaneous reaction channels of spin crossover and ligation to form coordinatively saturated species introduces an efficient pathway circumventing this spin barrier. It also supports the idea that the high density of electronic excited states and the relatively large amount of excess energy available in the system determine the course of the excited-state dynamics, rather than spin selection rules alone 5,6 . Fast ligation could be facilitated along the singlet pathway, confirming the general notion that solvent-stabilized metal centers form fast 3, 4, 11 and consistent with the observation of unsaturated carbonyl Cr(CO) 5 forming a solvent complex in alcohol solution within 1.6 ps 26 . An alternative proposal 20 for Fe(CO) 5 involves concerted exchange of CO and EtOH on the time scale of ligand dissociation of 100-150 fs. This would also proceed along a singlet pathway and in agreement with our results, as the temporal resolution of our measurements is not sufficient to distinguish between this concerted and the alternative sequential process. Revealing in detail 8 the influence of solvent-solute interactions will have to be the subject of future studies, which could also explore whether the structure of the solute prior to dissociation 20 influences the excited-state branching ratio between the different pathways.We find that the ligation capability of Fe(CO) 4 is mostly determined by its d σ * LUMO, which receives σ donation from occupied CO or ethanol ligand orbitals. Population of the antibonding d σ * orbital in excited singlet ( 1 B 2 ) and triplet ( 3 B 2 ) Fe(CO) 4 impedes σ donation from ligands (see sketches in Figure 3), explaining the inertness of these species against ligation; this problem is absent in the ligation channel that produces coordinately saturated species. Establishing this correlation of orbital symmetry with spin multiplicity and reactivity 27 is enabled by the atom specificity with which x-ray laser based femtosecondresolution spectroscopy can explore frontier-orbital interactions. This ability gives unique access t...

Photoaquation Mechanism of Hexacyanoferrate(II) Ions: Ultrafast 2D UV and Transient Visible and IR Spectroscopies

“…The studies on the photo-induced dynamics of Fe(CO) 5 in EtOH suggest formation of the 1 [Fe(CO) 4 (EtOH)] complex, and the role of the spin state of the Fe(CO) 4 product is still debated. Wernet et al 26 reported a singlet complexation on sub-picosecond time scales, which was ascribed to a barrier-less bimolecular reaction where steric effects such as ethanol reorientation and concomitant hydrogen-bond breaking are absent or can easily be overcome. 27 This fast photosubstitution is in line with reports of CO-ligand substitution of [Cr(CO) 4 (bpy)] by solvent molecules, 28-32 from a vibrationally "hot" excited state, alongside relaxation into two lower-lying unreactive states.…”

“…[20][21][22][23][24] More recently, ultrafast studies on the photochemistry of Fe(CO) 5 in solution have been pushed into the x-ray domain and an Fe K-edge x-ray absorption study was reported by Rose-Petruck and co-workers, 25 while Wernet et al 26,27 implemented femtosecond Fe L 3 -edge resonant inelastic xray scattering (RIXS) at the x-ray Free Electron Laser in Stanford. RIXS, which is a variant of x-ray emission spectroscopy, is a sensitive probe of the spin state of molecular systems.…”

X-ray absorption spectroscopy using a self-seeded soft X-ray free-electron laser

“…The ultra-bright femtosecond (fs) x-ray pulses produced at XFELs allow not only the studies of ultrafast dynamics [2][3][4][5][6] and materials in extreme short-lived conditions [7], but also they can probe a sample faster than the onset of x-ray damage [8][9][10][11]. The seeded FERMI FEL at Elettra Synchrotron Trieste, Italy (~12-300 eV) also allows well-controlled experiments based on nonlinear techniques [12] and coherent control [13] mainly in the vacuum ultraviolet (VUV) spectral region.…”

Attosecond electron pulse trains and quantum state reconstruction in ultrafast transmission electron microscopy

“…Similarly, electron microscopy explores the use of shaped electron beams exhibiting particular spatial symmetries 5 or angular momentum 6,7 , and novel measurement schemes involving quantum aspects of electron probes have been proposed 8,9 . Ultrafast imaging and spectroscopy with electrons and x-rays are the basis for an ongoing revolution in the understanding of dynamical processes in matter on atomic scales [10][11][12][13] . The underlying technology heavily rests on laser science for the generation and characterization of ever-shorter femtosecond electron 10,14 and xray [15][16][17] probe pulses, with examples in optical pulse compression 18 and streaking spectroscopy [19][20][21] .…”